Ikrus, Well, you don't want to lose money for doing someone a favor.
Depends on whether you are including the cost of the proteins or if they simply are essentially "renting" you and your smoker.
If you have a place close by that sells smoked meats, (at best) you can mirror whatever they charge and possibly give a discount of some sort. Generally, that includes the cost of the protein, supplies, consumables, etc. and still allows you to make a possible profit.
For example, a local place close to home, (Rudys' Country Store) Charges $4.99 for a 1/2 chicken, $6.99 Brisket./1/2 Lb., Sausage $3.69/link. I normally charge a little less when I do it. For company functions, I simply charge whatever it costs me, no profit at all.
If they are just "renting" you and the smoker, you generally would charge a flat rate.
If I am smoking some things, I often ask if my neighbors ( in advance) if they are interested in " tossing something in" to fill the smoker. In that case, since I am already footing the bill for the supplies, I simply tell them to toss in a couple of bucks for future fuel costs. Most of the time, we just hang out and it is about a break even scenario.
I no longer have a license, so most of my efforts are for family, friends, and functions at work. That being the case, I just try to make sure I don't actually lose my shirt and still enjoy the time spent with family, friends, and the occasional co-worker.
If you are considering doing it for a business, that is a different case altogether.
Either way best of luck to you!